This invention is generally directed to the function of providing a solid-state RF bypass for tower-mounted low noise amplifiers (LNA) when DC power to the LNA fails.
While the invention may find other applications, the invention is described herein with specific reference to the problem of bypassing amplifiers such as low noise amplifiers in a tower-mounted active antenna system, such as a base station for wireless communications. Generally speaking, such base stations employ a number of low noise amplifiers, which in the case of so-called active antenna systems are mounted in the tower adjacent the radiating elements or antennas. On the receive side of such an installation, a low noise amplifier is typically employed at the tower top for improving signal to noise ratio and providing an amplified signal to other base station equipment which may also be tower mounted, but which is more typically mounted in a base station enclosure at the base of the tower. Since access to the tower mounted equipment requires a worker to actually climb the tower, it is considered desirable to arrange for various fail-safe devices or modes of operation of the tower mounted equipment.
One such mode of operation is an automatic bypass capability for bypassing the low noise amplifier (LNA) when a failure mode of the amplifier is detected. The invention provides a bypass upon loss of the DC bias current supplied to the tower mounted equipment, which might occur for a number of reasons. Generally, electromechanical relays have been required to perform the bypass function when DC power is lost.
In cases of a defined failure mode of the LNA, the LNA may be bypassed, feeding the received signals directly from the antenna to the base station equipment, typically at the enclosure at the base of the tower. While this may degrade the signal to noise ratio somewhat in the absence of the amplifier, it is usually considered preferable to the signal degradation which may take place when the amplifier is in a failure mode. Therefore, it is an object of the invention to provide a switching arrangement which can bypass an LNA which is in a predefined failure mode in the absence of a control bias.
Heretofore, relatively expensive and cumbersome RF or microwave-compliant relay circuits have been used to accomplish this bypass function. The switching device or relay must meet stringent requirements to assure signal quality, including a relatively low insertion loss and relatively high isolation. In addition, a relay system is subject to the usual shortcomings of mechanical switching systems, including relatively low speed, high expense, relatively large size and susceptibility to various mechanical failure modes.
When failure of a Tower Mounted Amplifier (TMA) occurs, it is necessary to bypass the Low Noise Amplifier (LNA) in order for the system to continue functioning. It is desirable that the bypass mechanism be inexpensive, low loss, and solid state in construction. Additionally, the design must function in the bypass mode with all DC input power removed.